Asphalt roads are made using asphalt mixtures. Asphalt mixtures contain mineral aggregate, natural or manufactured sand and hot asphalt (called a binder) blended together, sometimes with other additives. The mix is transported to a road construction site, and the asphalt mixture is placed on a prepared road bed in a uniform layer. The layered asphalt mix is compacted in place until a certain prescribed density is obtained. After cooling, the compacted road surface becomes a long-lasting, stable surface capable of supporting a wide range of vehicles.
Asphalt Failure
Roads and highways constructed from asphalt mixtures are designed by pavement engineers to resist traffic loads and environmental extremes usually for 20 years of pavement performance without failure. The design process starts with the determination of a layered load bearing structure consisting of unbound layers as well as bound (asphalt mixture) layers. The mineral aggregate with a pre-designed gradation is then mixed with asphalt binder with or without polymers to produce a material capable of resisting traffic loads and environmental forces. However, pavements do fail for a variety of reasons. There are five areas of potential pavement distress that can lead to pavement failure: fatigue cracking, rutting, thermal cracking, friction, and moisture susceptibility. All of these distresses can result in loss of pavement performance thereby reducing the life cycle costs. Highway engineers specify mix designs that maximize the life of the pavement while minimizing the life-cycle cost of the road surface.
Repeated slow moving traffic loads cause accumulation of longitudinal (in the direction of traffic) wave like undulations called Rutting (or permanent deformation). Rutting deformation may accumulate in both the bound (asphalt mix layer) and the unbound layers of the pavement structure. This can occur for many reasons, including weak subgrade, and an unbound base course and problems with asphalt mixture design. The focus of this effort is permanent deformation, fatigue cracking, low-temperature cracking, moisture damage and friction pavement performance issues caused by asphalt mix problems. Permanent deformation is caused by consolidation and/or lateral shear deformation of the material in the asphalt mix layer under slow moving traffic. Shear deformation of the asphalt mixture generally occurs in the top 100 mm of the pavement surface, but it can occur deeper in pavement if properly designed asphalt mixture is not used. The mechanism by which rutting occurs is a combination of densification (decrease in volume and, hence, increase in density of the pavement) and shear deformation. Research literature suggests that rutting is mainly caused by deformation shear flow rather than densification volume change.
Fatigue cracking—often called alligator cracking—manifests itself as longitudinal cracks in the pavement surface that resemble the back of an alligator skin. This cracking initiates at the bottom of the bound asphalt mix layer due to repeated tensile strains caused by traffic loads (both slow and fast moving). The mechanism of fatigue cracking is similar to rutting in which the strains are accumulated to cause distress. The tensile strains accumulate to cause cracks when the material's ability to withstand the tensile strains is exceeded. Fatigue cracking is often associated with loads which are too heavy and/or frequent and exceed the pavement design limits. The cracks allow water to drain to the lower unbound layers and when coupled with inadequate pavement drainage can make the problem worse. Upper asphalt mix layers experience high strains when the underlying layers are weakened by excess moisture and can fail prematurely from fatigue. Fatigue cracking can lead to the development of potholes (asphalt mix pieces separate from each other when the cracks connect and are dislodged from the pavement surface). Fatigue cracking generally occurs on thin pavements where are potholes are more severe.
Fatigue cracking is generally considered to be more of a structural problem than just a material problem. However, if the asphalt mixture is adequately designed using high tensile strain resistant polymer or crumb rubber modified binders that can flex without cracking, even weak pavement structures will perform adequately. Fatigue cracking is caused by an interaction of number of pavement factors: heavy traffic loads and poor subgrade drainage combined with poorly designed, high-deflection pavements.
In the past, fatigue cracking was generally accepted to initiate from the bottom and migrate toward the surface, but fatigue cracks have also been observed starting at the surface and migrating downward in asphalt pavements. There is another form of cracking that manifests itself as transverse cracks (transverse to the traffic direction). These are called thermal or low-temperature cracks. Low temperature cracking of asphalt pavements is attributed to tensile strain induced in hot mix asphalt as the temperature drops to some critically low level. Low temperature cracking is caused by low pavement temperatures and influenced by traffic loads. As the pavement shrinks due to rapid drop in temperature, tensile strains build within the layer. At consistently spaced locations along the pavement, the tensile stress exceeds the tensile strength and the asphalt layer cracks. Low temperature cracks often occur from a single low temperature event, but they can also result from the cumulative effect of many freeze-thaw cycles. These cracks start at the surface and work their way downward. The mixture low-temperature strength, which is primarily related to the properties of the asphalt binder, is probably the greatest contributor to low-temperature cracking.
Moisture damage is also considered to be a major distress affecting asphalt pavements. The mechanism of moisture damage failure is creation of high tensile strain pockets in the asphalt layer due to loss of fine aggregate in the underlying unbound layers caused by poor drainage of the water ingresses from fatigue cracks. Moisture damage may also result from binder stripping from the mineral aggregate due to failure of adhesive bond or dissolution of the binder film or both.
Asphalt binder is produced by fractional distillation of crude oil. The lighter molecules of the crude oil are separated and used to manufacture such products as aviation fuel, gasoline and diesel fuel. Heavier lubricants are also removed, as are heavier fuel oils, until the leftovers are a near-solid pitch-like bottom byproduct. Due to high cost of gas used in cars and airplanes, refiners prefer distilling crudes that have abundance of lighter molecules thereby reducing the heavier bottom content that is turned into asphalt. New sources of crude oil like heavy, sour crudes from the Alberta Tar Sands and Orinoco River Valley are finding their way into the markets, thus modifying the crude mixes used to produce refined oil products. Along with changes in refining and increasing crude costs, asphalts today are experiencing a gradual decline in quality. In general, the asphalt of today does not function optimally in producing a lasting road surface unless it is modified as a physical composite or a chemically formulated material. Two of those methods of modifying/formulating asphalt binder are (1) asphalt modification with elastomeric polymers and (2) asphalt modification with recycled crumb rubber.
Testing of Asphalt Binders
Given the extended life of some asphalt roads, it may take fifteen or more years to observe the effects of a new additive or mix design in the field. In order to reduce the time required to assess the performance of any specific mix designs, the industry is constantly developing and deploying lab testing methods designed to forecast the expected future performance of a mix design. Some of the more prominent testing procedures in common use in the US involve evaluations of the binder performance or evaluation of the actual asphalt mix design. Frequently used binder tests include: (1) PG Grading as part of the US Superpave Program (AASHTO M320), (2) the Multiple Stress Creep Recovery (MSCR) Test (ASTM D7405) and (3) the m Value BBR Test (AASHTO M313 and ASTM D6648). Frequently used mix performance assessment laboratory tests include: (1) Hamburg Wheel Tracking Test (AASHTO M323 and AASHTO M332) and (2) Mixture Fatigue Testing (AASHTO T321 and S-VECD testing), Dynamic Modulus and Flow Number Testing using the AMPT (AASHTO TP79), Low-temperature cracking resistance testing for asphalt binders using the BBR (AASHTO T313) and the Direct Tension Test (AASHTO T316) and for mixtures using the TSRST test.
PG Grading as a part of the US Superpave Program is based on the idea that asphalt binder properties should be related to the climatic and traffic conditions under which they are used. PG graded asphalt binders are selected to meet expected climatic conditions as well as aging considerations with a certain level of reliability. Therefore, the PG system uses a common set of tests to measure binder physical properties that can be directly related to field performance of the pavement at extreme temperatures. A binder graded by the PG system should meet all specified criteria by this common set of tests.
The Rotational Viscometer (RV) test is used to evaluate the pumping ability of the asphalt binder at the asphalt plant. The Dynamic Shear Rheometer (DSR) tests with different levels of aging are used to evaluate the binder rutting resistance properties at high service temperature and fatigue resistance at intermediate service temperature. The Bending Beam Rheometer (BBR) and Direct Tension Test (DTT) with short plant induced and long term in service aging are used to evaluate the binder properties under low temperature, which are indicative of the likelihood that the binder will crack under stress at low temperatures. The Rolling Thin Film Oven (RTFO) aging is used to simulate the short-team aging of the asphalt binder during the construction phase, including the mixing, silo, transportation and lay down etc. The Pressure Aging Vessel (PAV) aging is used to simulate the long-term aging of the asphalt binder after the AC pavement is in service about 8 to 10 years.
The PG grading system is based on climate, so the grade notation consists of two portions: high and low pavement service temperature. The major concern for high temperature performance is rutting, which typically takes time to accumulate; therefore, an average of 7 day maximum pavement temperature is used for describing the high temperature pavement climate. On the low temperature side, thermal cracking can happen during one really cold night; therefore the minimum pavement temperature is used for describing the low temperature climate. For both high and low temperature grades, PG is reported in 6° C. increments. The average 7 day maximum pavement temperature typically ranges from 46 to 82° C., and minimum pavement temperature typically ranges from −46° C. to −10° C.
A binder identified as PG 64-10 must meet performance criteria at an average 7 day maximum pavement temperature of 64° C. and also at a minimum pavement temperature of −10° C. The maximum pavement temperature is typically higher than the air temperature by about 20° C. since the dark color pavement absorbs the heat and retains it. The maximum pavement temperature is typically measured at about 1 inch below the pavement the surface. However, the minimum pavement temperature occurs on the surface of the pavement and is equal to the air temperature.
The common minimum reliability used is 98%, so that means when the PG 64-10 binder is selected, the asphalt binder in the AC pavement should perform satisfactorily under normal traffic condition at the location where the extreme pavement temperatures are within the range of −10° C. and 64° C. throughout its service life with a minimum 98% confidence level. Where the traffic condition is not typical, such as the really heavy traffic like an interstate highway, or slow traffic like a bus stop or intersection area, one or two grades stiffer asphalt binder may be used to help prevent a rutting problem.
The Multiple Stress Creep Recovery (MSCR) test is a prominent and widely used improvement to the Superpave Performance Graded (PG) Asphalt Binder specification. This new test and specification provide the user with a new high temperature binder specification that more accurately indicates the rutting performance of the asphalt binder and is blind to modification. A major benefit of the new MSCR test is that it eliminates the need to run tests such as elastic recovery, toughness and tenacity, and force ductility, procedures designed specifically to indicate polymer modification of asphalt binders. A single MSCR test can provide information on both performance and formulation of the asphalt binder.
The MSCR test uses a creep and recovery test concept to evaluate the binder's potential for permanent deformation. Using the Dynamic Shear Rheometer (DSR), the same piece of equipment used today in the existing PG specification, a one-second creep load is applied to the asphalt binder sample. After the 1-second load is removed, the sample is allowed to recover for 9 seconds. The test is started with the application of a low stress (0.1 kPa) for 10 creep/recovery cycles then the stress in increased to 3.2 kPa and repeated for an additional 10 cycles.
The material response in the MSCR test is significantly different than the response in the existing PG tests. In the PG system, the high temperature parameter, G*/sind, is measured by applying an oscillating load to the binder at very low strain. Due to the low strain level, the PG high temperature parameter doesn't accurately represent the ability of polymer modified binders to resist rutting.
Under the very low levels of stress and strain present in dynamic modulus testing, the polymer network is never really activated. In the existing PG specification, the polymer is really only measured as a filler that stiffens the asphalt. In the MSCR test, higher levels of stress and strain are applied to the binder, better representing what occurs in an actual pavement. By using the higher levels of stress and strain in the MSCR test, the response of the asphalt binder captures not only the stiffening effects of the polymer, but also the delayed elastic effects (where the binder behaves a bit like a rubber band).
The relationship between rutting and the results from MSCR testing was verified by FHWA through their ALF process and with test sections on I-55 in Mississippi. In 1996, Mississippi DOT (Department of Transportation) built several test sections on I-55. Multiple modifiers were used in the sections, including SBS, SB, SBR, and crumb rubber, with a control section of unmodified (neat) asphalt binder. Rutting was monitored for six years. The findings from both studies indicated that the MSCR parameter, Jnr, correlated much better to rutting then the PG parameter, G*/sin d.
The MSCR test does a better job of identifying the rut resistance of both neat and polymer modified binders, but some highway agencies still want to make sure that the polymer is in the binder for other purposes such as crack resistance and durability. Here the MSCR test provides great improvements over the existing tests like the elastic recovery and toughness and tenacity. Data from the exact same sample from the MSCR test that was used to do high temperature grading provides information on the polymer modification as well. The one test provides the high temperature grade and quality of polymer modification eliminating the need to run additional tests like elastic recovery on additional samples. The compliance value Jnr from the MSCR test provides the rut resistance and the amount of recovered strain from the test identifies the presence of polymer and also the quality of the blending of the polymer in the binder.
AASHTO M320 currently does not have a specification on items such as Elastic Recovery or any of the currently used SHRP+ tests. In keeping with their current practice, no actual specification was developed for the % Recovery in the MSCR test. Recommendations on minimum MSCR % Recovery are part of the TP 70 test procedure for MSCR.
The Multi-Stress Creep and Recovery Test was developed to replace the existing RTFOT DSR high temperature Superpave requirement. It has been to shown to be far more discriminating in identifying the rutting potential of both modified and neat binders. The MSCR has been used to test and successfully rank neat, SBS, SB, Elvaloy (terPolymer), CRM, latex and chemically modified binders. The same simple test procedure to determine the high temperature test can also be used to evaluate the presence of polymer modification in the binder eliminating the need to run other time consuming, less discriminating test methods. Although jurisdictions can use their own MSCR threshold standards, many call for MSCR recovery values in excess of 35% in order for a pavement to test as acceptably rut resistant.
m-Value from BBR test and phase angle value from the DSR test—Low temperatures cause an asphalt binder to become very stiff with low resistance to movement. Tensile stresses from movement can exceed the tensile strength of the asphalt pavement, leading to cracking. To prevent low temperature cracking, the PG binder specification uses a maximum creep stiffness (S) of 300 MPa. The S value is determined through a procedure involving use of a bending beam rheometer (BBR). A second BBR measurement is the m value. The m value represents the rate of change in the creep stiffness in a binder sample versus time. The m value is obtained by measuring the stiffness of an asphalt binder beam at several times after application of a standard load to the beam. The m value is the slope of the curve of log stiffness versus log time, and the slope expresses the rate of binder stiffness change over time in cold conditions. A high m value is desired because a pavement in cold conditions will contract, and a less stiff binder in cold temperatures will resist cracking. The PG binder specification requires that the m value be greater than or equal to 0.300 at 60 s loading time at low service temperature+10° C. A lower phase angle measured using the DSR also provides additional support to the elasticity of the mix and impacts the rutting and cracking performance in the field as shown by numerous research studies.
The Hamburg Wheel Tracking Test examines the susceptibility of asphalt surfaces to rutting and moisture damage. The test uses a steel wheel with weight that rolls over the sample in a heated water bath. A designated number of passes (20,000 is common) are performed on the sample. The rut depth is measured by the machine periodically, usually every 20, 50, 100 or 200 passes. 20,000 passes can take around 6.5 hours whereas the entire test can take around 3 days. Several analytics are examined with the Hamburg Wheel Tracking Test including post-compaction consolidation, creep slope, stripping inflection point, and stripping slope.
Studies have found that there is good correlation between the Hamburg test and field performance but it has also been determined that the test can fail to differentiate between some mixtures.
Fatigue Testing of binders and mixtures—Several test methods are under development to assess the fatigue resistance of asphalt binders. The most promising among them is the DENT test being developed by FHWA at the TFHRC research labs. Recently AASTO has approved it as a provisional standard. Fatigue distress of the pavement is mostly an asphalt mixture material fatigue resistance issue more than the binder's fatigue resistance. With that in mind, the S-VECD pull-pull fatigue testing is currently being developed to test mix fatigue resistance. The bending beam fatigue test (AASHTO M321) is also widely used to assess fatigue resistance of asphalt mixes. The pull-pull test is however easier to use and produces results with relatively less time and effort as compared with the bending beam mix fatigue test.
Polymer Modification of Asphalt
Polymer modification of asphalt involves the addition of mostly elastomeric polymers (SBS, SB, SBR etc.) to asphalt binders for the purposes of creating an asphalt binder capable of resisting tensile strains due to cross-linking of polymer molecules. The two lead additives for polymer modification are SBS (Styrene Butadiene Styrene) and SBR (Styrene Butadiene Rubber). These additives are added in hot asphalt liquid, heated and mixed with a shearing mixer at an asphalt terminal before transport to an asphalt pavement manufacturer for inclusion in various pavement designs.
About 10-15% of all domestic hot and warm mix asphalt is modified with polymers. Virtually all polymer modified asphalt (PMA) uses SBS (85% of the market) or SBR (10% of the market). Modified asphalt pavements are typically used in high-stress applications like interstate expressways and roads supporting heavy industrial traffic.
The SuperPave system classifies asphalt binders based on their ability to survive varying use climates and varying loadings. Standard binders would have a Performance Grading (PG) classification of 64-22. Polymer modified binders typically are designed to meet a PG 76-22 standard, which generally means they will resist rutting and cracking better than a PG 64-22 binder. Polymer modification of asphalt can extend road life by 35% or more.
Rubber Modified Asphalt
The primary source of crumb rubber in the US is from recycled used auto and truck tires. The tires are chopped into smaller pieces, the belts are removed, and the chipped tires are reduced to progressively smaller crumbs. In 1990, the federal government mandated the use of crumb rubber as a partial replacement for asphalt binder. Asphalt binder with more than 5% rubber content becomes more resistant to rutting, and can resist cracking as well. Early experiments with rubber additions to asphalt did not go well, and there were a number of quality-related problems in the industry. The federal mandate was rescinded after a few years. Several states continued to work on the problem, and Arizona developed a process for rubber addition that was successfully deployed. The Arizona “Wet Method” modified binders with as much as 15 to 20% crumb rubber, and the resultant roads were very effective in reducing rut and crack problems, not to mention other benefits. The wet process has a few drawbacks, however. The rubber has to be cooked or digested into the binder for an hour or more at high temperatures before use. With higher levels of rubber additions, the hot mix asphalt is very sticky and difficult to lay and compact. Modification is typically performed either at an oil terminal or at a larger asphalt production facility. Blending and cooking processes require a separate processing, pumping and storage tank, which can be expensive. If the modified asphalt is stored for any length of time, it has to be agitated in order to keep the crumb rubber properly diffused in the mix. Finally, the cooking process tends to require more energy in order to produce a ton of hot mix. In spite of those drawbacks, the wet process produces excellent roads that are as durable (or more durable) than polymer modified asphalts.
When a CRM asphalt binder is subjected to some of the binder and mix tests discussed earlier, the results are very encouraging. With the addition of 10% or more rubber, the modified binders typically showed PG ratings in the 82-22 range. The mix designs had no trouble passing mix testing requirements set by various DOTs (Departments of Transportation), and depending on the starting binders used, MSCR values of more than 35, and passing m values were common. Both lab and field results suggest that both wet mix asphalt and the dry process or plant mix asphalt are both viable methods for asphalt mixture modification for paving and other related applications.